Two Conceptions of Earth History:

Catastrophism

Uniformitarianism

Assumption: We Can Use Cause And Effect to Determine Causes of Past Events

Finding: Earth History Dominated by Small-scale Events Typical of the
Present.

Catastrophes Do Happen But Are Uncommon

Fossils - Remains of Ancient Plants And Animals, Evidence of Life

Commonly Preserved: Hard Parts of Organisms

Bones

Shells

Hard Parts of Insects

Woody

Material

Rarely Preserved: Soft or Easily Decayed Parts of Organisms

Internal Organs

Skin

Hair

Feathers

Types of Fossils

Original Material

Casts & Molds

Replacement (Petrified Wood)

Carbonized Films (Leaves)

Footprints, Tracks, Etc.

Pseudo-Fossils: Look Like Fossils But Aren't

Dendrites

Concretions

Fossils Occur Almost Exclusively in Sedimentary Rocks

Heat of Melting or Metamorphism Would Destroy Almost Every Type of Fossil

Rare Exceptions:

Some Fossils in Low-grade Metamorphic Rocks, Trees

Buried by Lava Flow

To Be Preserved, Organisms Have to Be:

Buried Rapidly After Death

Preserved From Decay

These Are Difficult Requirements! Probably Only One Organism in Millions Ever
Gets Fossilized

Good Index Fossils

Abundant

Widely-distributed (Global Preferred)

Short-lived or Rapidly Changing

Correlation

If we have index fossils at widely separated locations, we can correlate rock
units. Short-lived but widespread fossils like D, E, and F are very useful. So
are first occurrences like B, last occurrences like C, and overlaps like C-D and
B-D.

However, it takes organisms time to expand their ranges, and the absence of a
fossil doesn't mean it the organism was never there. We find fossil D on both
sides of the gap, but E only on the left and F only on the right. E must
correlate somewhere below D and F somewhere above, but we don't know exactly
where.

Mass Extinctions

Permian

Trilobites

Many Brachiopods

Many Tree Ferns, Scale Trees

Worst in Earth History - 90% of species

Cretaceous

Dinosaurs

Ammonites

Moderately Bad - only 60% of species

Three others as bad or worse

Causes ?

Climate Change

Disease

Mountain-building

Sea Level Change

Competing Organisms

Over-specialization

Catastrophe

Volcanism

Meteor Impact

Were the Dinosaurs Failures?

We often use the term "Dinosaur" to denote something clunky or obsolete, but
consider:

Written History Lasted 5000 Years

The Dinosaurs Lasted 150,000,000 Years

For Every Year of written history:

The Dinosaurs had 30,000 Years

For Every Day of written history:

The Dinosaurs had 82 Years - a long human lifetime

For Every Minute of written history:

The Dinosaurs had 3 weeks

For Every Second of written history:

The Dinosaurs had 8.3 hours

Two Kinds of Ages

Relative - Know Order of Events But Not Dates

Civil War Happened Before W.W.II

Bedrock in Wisconsin Formed Before The Glaciers Came

Absolute - Know Dates

Civil War 1861-1965

World War II 1939-1945

Glaciers Left Wisconsin About 11,000 Years Ago

Relative Ages

Superposition - Young Events Leave Traces Behind of Older Rocks

Young Rocks Laid Down on Older Rocks

Intrusions Are Younger Than The Rocks They Intrude

Folds And Faults Are Younger Than The Rocks They Occur in

When Rocks Are Tilted, It is Possible to Determine Which Way Was
Originally Up

Absolute Ages

Early Attempts

Bible

Add up Dates in Bible

Get an Age of 4000-6000 B.C. For Earth

John Lightfoot and Bishop Ussher - 4004 B.C. (1584)

Too Short

Today, it's fashionable to make fun of Lightfoot and Ussher, but they were
being the best scientists they could be given the state of knowledge at the
time (note this is before most people believed the Earth goes around the Sun
and 50 years before Galileo ran into trouble).

Salt in Ocean

Rivers bring dissolved solids to ocean

If we know rate salt is added, and how much salt is in ocean, can find age
of oceans.

Gave age of about 100 million years.

Problems

Is rate at which salt is added constant?

How much salt leaves ocean?

Sediment Thickness

Add up Thickest sediments for each period

Estimate rate of deposition to find age

Problem: rates of deposition very variable!

Indicated ages of at least 100 million years

Age of The Sun

One of the Great Scientific Controversies of the 19th Century

If sun gets its heat from burning, could only last 10,000 years or so.

Best That 19th century astronomers could guess was that sun was slowly
contracting.

Problem: only 30 million years ago, sun would have extended out to earth's
orbit!

Geologists wanted more time, but you can't fight the laws of physics...

We now believe sun gets its heat from nuclear reactions. These release
enough energy to keep sun going for billions of years

The Geologists were right after all.

Score One For My Team

Absolute Ages

Use Radioactive Decay to Date Rocks.

Radioactive atoms give off particles and change into different types of
atoms.

Radioactive atoms decay at a certain rate. They have half lives. After
each half life, half of the atoms will have decayed.

Half-life

Example I

You have $20 in pennies (2000)

Once a minute, you flip one, and if it comes up heads you put it aside

Someone can figure out how long it's been since you started, if they know
how many pennies there were originally, how often you flip, and how many heads and tails there are.

Example II

You have a large bowl full of jelly beans. You like the green ones but not
the red ones.

Every time you dip into the bowl and get a green bean, you eat it.

Every time you dip into the bowl and get a red bean, you put it back.

The number of green beans you eat is large at first but tapers off.
However, it may take a very long time to get that last green jelly bean.